This invention relates to time base error correcting apparatus and, more particularly, to such apparatus wherein, in the event of a dropped out picture element, a substitution therefor is made by replacing the dropped out element with a picture element disposed in the same relative location in the immediately preceding line interval.
When video signals are recorded on a magnetic medium, such as magnetic tape by a VTR, time base, or frequency and/or phase errors may be introduced into the video signals during playback because of various factors. For example, the record medium may expand or contract after the video signals have been recorded thereon. Also, the speed with which the record medium is transported in the playback device may differ slightly from its speed during recording. Similarly, there may be a slight variation between the rotary speed at which the heads scan the record medium during a signal recording operation and during a signal playback operation. All of these possible variations result in time base errors which appear as undesired effects in the ultimately reproduced video picture. These observable effects are perceived as jitter, brightness distortion, improper color display and the like. If the video signal which is played back from, for example, a VTR, is to be used in connection with the transmission of a television broadcast, or is to be merged with "live" broadcast information, it is important that these time base errors be corrected.
One example of a time base corrector for use with video signals is described in U.S. Pat. No. b 3,860,952. In this time base corrector, incoming video signals are converted from analog form into digital form and are temporarily stored in a digital memory. The digitized video signals are written into the memory at a write-in clock rate which varies in accordance with detected time base errors. The stored, digitized video signals are read out at a standard, fixed read-out clock rate, whereby the read out video signals are generally free of significant time base errors. Then, the read out digitized video signals are reconverted back into analog form.
In the time base correctors of the aforementioned type, if a drop-out condition is detected in the video signal that is written into the memory, the dropped out portion is replaced by a similar portion that had been present in the video signal two line intervals prior to the line interval containing the drop-out condition. This two-line interval delay (referred to herein as a 2H delay, where H is equal to one horizontal line interval) had been thought necessary because of the inherent characteristics of the NTSC color video signal. That is, the phase of the chrominance subcarrier during each odd line interval is opposite to the phase of the chrominance subcarrier during each even line interval. In order to maintain a proper phase relation in the event that a dropped out portion of a line interval is replaced by a previous, non-dropped out portion, the replacing portion should have the same subcarrier phase as the replaced portion. Hence, the replacing portion had been obtained from the line interval that occurred at the 2H delayed time. Unfortunately, the information contained in the replacing portion at the earlier 2H time may represent a completely different picture content than represented by the portion which it is replacing. This can result in a degradation of the video picture ultimately reproduced from this drop-out compensated video signal.
Another disadvantage of time base correctors of the aforementioned type is that they do not necessarily take velocity errors into account. The velocity error of a video signal is the time base error which may exist over a substantial portion of a horizontal line interval, which error is not detected until the completion of that line interval and, thus, may not be taken into account, or corrected, during that line interval. That is, in typical time base error correcting systems, the write-in clock signal has its frequency synchronized with the actual horizontal synchronizing pulse that is included in the played back video signal, and its phase is synchronized with the phase of the burst signal which also is included in the played back video signal. In accordance with the NTSC standard for television signals, both the horizontal synchronizing pulse and the burst signal are provided at the beginning of a horizontal line interval. Consequently, the synchronizing of the write-in clock signals to the incoming horizontal synchronizing and burst signals necessarily is achieved at the beginning of the horizontal line interval. Of course, if there is a time base error, or velocity error, in the video information portion of the horizontal line interval, there will be a phase shift between the write-in clock signals and the video signals. Since the digitized video signals are read out from the memory in the time base corrector at a standard, fixed read-out clock rate, the aforementioned phase shift during the write-in operation is not compensated during the read-out operation because of the use of such a fixed read-out clock rate. Consequently, the resultant video signal which is read out from the memory in the time base corrector may include velocity errors which are not compensated.
Although there have been proposals to detect and correct for velocity errors in time base correctors, such proposals still carry out drop-out compensation on the basis of substituting a portion of the video signal, delayed by 2H from the line interval containing the drop-out. For example, in copending Application Ser. No. 817,662, assigned to the assignee of the present invention, a time base corrector is disclosed which includes a velocity error detector and a velocity error correcting circuit. The velocity error is detected by comparing the phase of the write clock pulses, that is, the clock pulses which are used to write the digitized video signal into the memory of the time base corrector, to the phase of the burst signals which are played back from the VTR. Any phase differential therebetween represents the shift in phase during a horizontal line itnerval of the played back video signal. This phase shift representation is the velocity error and is stored in a velocity error memory device at a storage location corresponding to the storage location in the memory at which the line of video signals which includes such velocity error is stored. When the stored line of video signals is read out from the memory at the constant read-out rate, the velocity error which is associated with that line of video signals also is read out of the velocity error memory. This read out velocity error signal is integrated over a line interval so as to represent a linearly changing velocity error over that line interval. The integrated velocity error signal then is used to phase modulate the read-out clock pulses, whereby the line of video signals is read out of the memory at a phase-modulated rate. It is thought that this phase modulation of the read-out clock pulses is a close approximation of the velocity error which had been included in that line of video signals originally; and, therefore, compensates for this velocity error.
Another example of a time base corrector including velocity error compensating circuitry is described in U.S. Pat. No. 4,065,787. In this patent, the velocity error signal is determined by subtracting the difference between the control voltage which is applied to a voltage-controlled oscillator in the write-in clock generator phase-lock loop when one line of video signals is received from the control voltage which had been applied to the voltage-controlled oscillator when the preceding line was received. This difference is stored in a velocity error memory location corresponding to the location in the memory at which the presently received line of video signals is stored, and is read out when that line of video signals also is read out. Here too, the read out velocity error signal is integrated so as to approximate the actual velocity error as a linear function. A similar velocity error compensating arrangement is described in the paper "Digital Time Base Correction" by Kitson, Fletcher and Spencer, International Broadcasting Convention Technical Paper, September 1974.
An improved technique for compensating velocity errors is described in copending U.S. application Ser. No. 911,286, filed May 31, 1978 now U.S. Pat. No. 4,165,524. In this improvement, the velocity error is assumed to vary in a non-linear manner throughout a line interval. This non-linear variation is approximated by interpolating the velocity errors which have been detected for the present, immediately preceding and immediately succeeding line intervals.
In the aforementioned velocity error compensators, since velocity error detection is based upon sensing the burst signals that are included in each line interval, this detection cannot be attained if drop-out occurs during the horizontal blanking interval, i.e., during the synchronizing information portion of a line interval. Consequently, the velocity error of two successive line intervals may not be corrected because the velocity error of the line interval that preceded the drop-out occurrence as well as the velocity error of the line interval that followed the drop-out occurrence cannot be detected. Furthermore, if a line interval whose velocity error cannot be corrected also contains drop-out in the video information portion thereof, drop-out compensation therefor may be less than satisfactory because the velocity error compensation that is attempted for the replacing portion will be based upon the velocity error of the replaced portion which generally differs from the velocity error of the replacing portion (i.e., the portion that was present 2H delayed time ago).